This invention relates to a novel battery charger, and more specifically relates to a novel small and inexpensive off-line battery charger employing polarized isolation capacitors for isolating the a-c source from the d-c output side of the device, with respective bypass diodes for each of the polarized capacitors and means for controlling the output d-c voltage and the battery charging current.
Small power supplies, for example, those commonly housed in an a-c plug-in adaptor for a small low power d-c appliance are well known. Such devices commonly have a two-winding transformer for providing low voltage and isolation between the a-c and d-c sides of the circuit and a bridge-connected rectifier and filter for producing a low voltage d-c output. Such devices can be used to operate small d-c appliances or to charge batteries or the like.
In order to reduce the size and cost of such power supplies, a-c capacitors and d-c coupling capacitors have been employed in place of a two-winding transformer. It is known, for example, from U.S. Pat. No. 4,412,278, dated Oct. 25, 1983 that the capacitors can be polarized d-c capacitors to provide a current-determining function. In these prior art circuits, the output d-c current is proportional to the input a-c voltage. Thus, if the system is used for different a-c input voltages, for example, 120 or 240 volts a-c, the output d-c current is different accordingly. Therefore, such a circuit cannot be used as a "universal" battery charger to charge a battery of given current capacity. Furthermore, in such prior art systems, the battery charging current is generally proportional to the input a-c voltage regardless of the battery voltage or its state of charge. This condition can cause damage to some batteries so, for this further reason, systems of this type cannot be universal battery chargers